The invention relates to a method for determining speed of terminal in a radio system, the method comprising sampling from a received signal at given intervals, calculation of a channel impulse response on the basis of the received signal and determination of a channel impulse response energy, and calculating a mean strength for a sampled signal within a given time window.
The present invention is applicable particularly to cellular radio systems and to other digital radio systems too, where terminals communicating with base stations moves across cell limits. Information about the moving speed of the terminal substantially facilitates the radio system resource management such as handover anticipation and power control optimization.
It is preferable, for example, in a system using small microcells and with them larger overlapping so-called umbrella cells that fast moving terminals communicate with the umbrella cells and slow moving or stationary terminals with the smaller microcells. In this way the number of handovers needed and the signalling load of the network can be decisively reduced.
In a typical cellular radio environment the signals between a base station and a subscriber terminal propagate on several routes between a transmitter and a receiver. This multipath propagation is mainly caused by signal reflections from surrounding surfaces. Signals travelling on different routes arrive at the receiver at different times owing to a different propagation delay. This holds true for both transmission directions. The multipath propagation of a signal can be monitored in a receiver by measuring the impulse response of the received signal, signals arriving at different times being visible as peaks proportional to their signal strength.
A known solution for determining speed of a moving terminal is to monitor the Doppler shift of the carrier wave frequency. However, this method is impractical as it requires a stable frequency reference source which is very expensive.
Another known method is described in the article by Doumi, T., Gardiner, J. G.: Use of base station diversity for mobile speed estimation, Electronic Letters, Vol. 30, no. 22, pp. 1835-1836. Base station antenna diversity is utilized in the method of the publication. The antenna providing the strongest signal level is always selected in diversity. An antenna duplex frequency is proportional to the Doppler shift on the basis of which an apparatus speed can be determined.
A method for measuring the apparatus speed when using antenna diversity is described in patent publication U.S. Pat. No. 5,396,645. In the method of the publication the received signal strength fluctuations are measured in connection with antenna diversity.
An object of the present invention is to enable terminal speed measuring without antenna diversity using a relatively simple and advantageous equipment.
This is achieved with the method of the type set forth in the preamble characterized by determining a mean energy of several consecutive calculated channel impulse responses, comparing instantaneous impulse response strength values with a calculated mean strength, and calculating how frequently an instantaneous strength moves above or below a mean value within a given time window, and by determining an apparatus speed on the basis of the number of transitions.
The invention also relates to a method for determining speed of terminal in a radio system, the method comprising the use of antenna diversity in signal reception, sampling from a received signal at given intervals, the determination of a signal instantaneous strength by measuring a signal strength of each sample, and calculating a mean strength for a sampled signal within a given time window in each diversity branch. The method of the invention is characterized by comparing instantaneous strength values with the calculated mean strength in each diversity branch and by calculating how frequently the instantaneous strength moves above or below a mean value within the given time window, and by determining an apparatus speed in each diversity branch on the basis of the number of transitions and by selecting as a speed value the speed value given by the diversity branch offering the best signal quality.
The invention further relates to a receiver in a radio system, the receiver comprising means for receiving a terminal signal with at least two diversity antennas, means for sampling a received signal at given intervals, means for determining a signal instantaneous strength by measuring a signal strength of each sample and means for calculating a mean strength for a sampled signal within a given time window, and means for determining the signal quality received by each antenna. The receiver of the invention is characterized by the receiver comprising means for comparing instantaneous strength values with a calculated mean strength and for calculating how frequently an instantaneous strength moves above or below a mean value within the given time window, means for determining an apparatus speed in each diversity branch, and means for determining the apparatus speed based on the number of transitions in each diversity branch and means for selecting as a speed value to be utilized the speed value given by the diversity branch offering the best signal quality.
The solution of the invention has several advantages. The method of the invention enables the fast enough estimation of the moving station speed as far as handover is concerned. The use of antenna diversity is not necessary for utilizing the invention, although the invention functions also in connection with diversity.
In order to avoid error measuring caused by signal noise a threshold value can be used in connection with the mean strength value in such a manner that when the instantaneous strengths are compared with the mean value the transition is not taken into account until the instantaneous value has moved across the mean value by a certain determined threshold value.